This invention relates to an optical transmission device for transmitting data between different apparatus having communication functions such as a sequence controller or a computer. More specifically, the present invention relates to a transmission controller interposed between a full duplex optical transmission line and a half duplex electrical transmission line.
Conventionally, a communication system is known wherein sequence controllers or computers disposed at various machining stations in a factory are connected to a communication line to effect mutual communications between them. In such communication system, mutual communications between the machining stations or between a control center and the individual machining stations are carried out. When transmission or reception of data is carried out at each of the machining stations, data interchange is sometimes carried out with optical coupling in order to eliminate a disturbance by noises or to facilitate transmission and reception of data to and from a moving body.
In such a communication system, an optical transmission line (including a spatial transmission line) exists in a part of the communication line. However, since the optical transmission line is constituted from a full duplex transmission line, any other electrical transmission line to be connected with the optical transmission line must necessarily be a full duplex electrical transmission line.
Meanwhile, data transmission between the different stations or between the control center and each of the stations can be sufficiently made by a half duplex transmission line due to an amount of data to be transmitted thereby. Accordingly, employment of the full duplex transmission line will make a waste of wires and may increase failures of breaking of wires or short-circuiting.
On the other hand, the optical transmission device interposed between the electrical transmission line and the optical transmission line includes a light emitting device for converting an electric signal into an optical signal, and a light receiving device for changing an optical signal into an electric signal. In such a device, dust, oil or the like may stick to a surface of an output window of the light emitting device or an input window of the light receiving device to deteriorate the optical transmittivity of the window. And also the optical axes of the light emitting device and the light receiving device may be brought out of alignment with each other. Consequently, accurate transmission of data may not be assured. In order to detect such an abnormal condition as described just above, the level of the optical signal (amount of light) received by the light receiving device is monitored. When the optical signal level becomes lower than a stable operation assuring level, a warning signal output is given. Such warning signal is transmitted also to the control center so that a warning lamp of the center may be switched on in response to the signal. To this end, a dedicated signal line for transmitting a warning signal must be provided between each machining station and the control center.